1. Field of the Preferred Embodiment(s)
This invention generally relates to oscillators which provide a stable reference source or frequency in computers or other electronic equipment. Specifically, there is a resonating device support structure and oscillator package which simultaneously provides mechanical and thermal isolation between a crystal resonating device and a circuit board. The support is enhanced to improve thermal isolation.
2. Description of the Related Art
Various devices are well known for providing a reference frequency or source. Such devices are called oscillators. The oscillator typically has a quartz crystal source and also has electronic compensation circuitry to stabilize the output frequency. Ovenized oscillators heat the oscillator to a uniform temperature to obtain a more stable output frequency. The oscillators have been packaged on various support structures and in housings such as metal cans.
In order to minimize the quantity of electric power consumed by the oven assembly, it is necessary to maximize the thermal path resistance between the heated object (the crystal resonator) and the ambient environment. In a typical embodiment, it is desired to maintain the crystal resonator at a specific temperature in the range 70 to 90 degrees Centigrade, while the ambient temperature can go as low as -40 to -55 degrees C.
The biggest cause of power consumption in an ovenized crystal controlled oscillator is the need to electrically maintain the crystal resonator at a temperature corresponding to the so-called turnover temperature, i.e. a temperature where the slope of frequency deviation versus temperature characteristic curve approaches zero. In accordance with Fourier's law of heat conduction, sufficient power must be provided to heat the resonator and its attached material structures to counterbalance the continuous heat loss through those supporting structures to the ambient temperature surfaces.
There is a current unmet need for a crystal support structure having a significantly higher thermal resistivity. The heat flux Q per unit time (power) is related to the temperature drop dT and thermal resistance .THETA. by the following Ohms law type relationship: Q=dT/.THETA.. Here, Q is analogous to electric current, dT is analogous to electric voltage, and .THETA. is analogous to electric resistance. In order for an OCXO oven to consume less than 1 mW continuous in a 100 degree C. thermal gradient, the thermal resistance between the crystal resonator and the ambient enclosure must be at least 100,000.degree. C./Watt.
Another disadvantage of many present art crystal support designs is that the support imparts a residual mechanical stress to the resonator which results in undesired frequency shifts of the resonator. If this stress and frequency shift changes slowly over time, the oscillator or filter is said to exhibit "aging". Aging in practical oscillators has been attributed to many effects. These effects may be broadly classified into three categories: 1) aging due to changes in the materials of the resonator itself, 2) changes in the environment surrounding the resonator, or the enclosure itself, which results in mass being deposited onto or removed from the resonator, 3) aging due to changes in the mechanical stress imparted to the resonator by the supports or enclosure.
One approach to indirectly measuring the effect of support structure induced stresses on the crystal resonator is to conduct the following experiment:
Thermally cycle the crystal in it's supports and housing. Another sample is used as a control group. If aging tests reveal a significant difference in the frequency drift over time between the experimental group and the control group, then it can be said that support induced stress on the resonator exists. A more rapid version of this experiment is known as cryoprocessing, is especially effective when Kovar, Invar and other austentite steel alloys are used for the supports, or in the baseplate. The existence of residual stresses can be indirectly measured by performing an experiment on two groups of sealed crystal resonators. The first group is not subjected to thermal cycling, the second group is thermally cycled from room temperature, down some low temperature (such as -150.degree. C.), then back up to a high temperature (such as +125.degree. C.), and then back to room temperature. In some instances, this temperature cycle may be repeated more than once. The rate of change of temperature is typically less than 0.5.degree. C./min. Aging tests are then performed on both the control group (non cycled) and experimental group (temp. cycled), and if there is a difference in the aging of the two groups, then it can be said that a stress relaxation occurred in the cryoprocessed group. It is desirable to have a support structure which does not require the additional screening or manufacturing step of cryoprocessing to insure a given aging characteristic.